كيمياء إشعاعية

اضمحلال النوى

الكيمياء الإشعاعية مجال كيميائي يعنى بدراسة العناصر المشعة. كما يعالج إنتاج وتعريف واستخدام مثل تلك العناصر ونظائرها. وقد أفادت الكيمياء الإشعاعية، علم الأثار وعلم الكيمياء الحيوية والمجالات العلمية الأخرى. وتستخدم التقنيات الإشعاعية الكيميائية في الغالب في مجال الطب للمساعدة في تشخيص المرض، وفي العديد من الدراسات البيئية.

الطريقة التقليدية: يتم حرق قطعة من الجسم وتحويلها إلى غاز ثاني أكسيد الكربون. تُطلق ذرات الكربون المشع (كربون 14) الموجودة في الغاز إلكترونات أثناء تَحوُّل الكربون المشع إلى نيتروجين 14. يقوم عدّاد الإشعاع بحساب عدد الإلكترونات التي تم إطلاقها، والتي تحدد محتوى الجسم من الكربون المشع.

الكيمياء الإشعاعية مجال كيميائي يعنى بدراسة العناصر المشعة. كما يعالج إنتاج وتعريف واستخدام مثل تلك العناصر ونظائرها. وقد أفادت الكيمياء الإشعاعية، علم الأثار وعلم الكيمياء الحيوية والمجالات العلمية الأخرى. وتستخدم التقنيات الإشعاعية الكيميائية في الغالب في مجال الطب للمساعدة في تشخيص المرض، وفي العديد من الدراسات البيئية.

يوجد قليل من العناصر المشعة في الطبيعة كالثوريوم واليورانيوم أما العناصر الأخرى فتنتج صناعياً، حيث يمكن إنتاجها بداخل أجهزة تُسمى معجلات الجسيمات، وذلك بقذف العناصر غير المشعة بجسيمات عالية الطاقة. كما يمكن جعل العناصر مشعة بتعريضها لأعداد كبيرة من النيوترونات داخل المفاعلات النووية. وتسمّى نظائر العناصر المشعة النويدات المشعة أو النظائر المشعة. وتُستخدم هذه النظائر بمثابة عناصر استشفافية في أنواع معينة من البحوث، وبالأخص في دراسة العمليات الأحيائية المعقدة. ويقوم هذا النوع من الدراسة، بتتبع النويدات المشعة، من خلال التفاعلات الكيميائية في الكائنات الحية. وتتم عملية التتبع هذه باستخدام عدّادات جايجر، والعدادات النسبية وأجهزة الكشف الأخرى.

الطريقة الحديثة: يُطلق معجل جسيمات ذرات مشحونة من قطعة صغيرة من الجسم إلى داخل مجال مغنطيسي. ويقوم المجال المغنطيسي بتحريك ذرات الكربون المختلفة جانباً وفصلها تبعاً لأوزانها. ومن ثم يقوم مكشاف بحساب ذرات الكربون 14 فقط، لتحديد محتوى الجسم من الكربون المشع.

ويتم إنتاج النويدة المشعة، بكميات صغيرة، ولهذا فهي تميل للتراكم على جدران الإناء الذي يحتويها قبل التمكن من استخدامها. ويتم منع حدوث هذه العملية بإضافة عنصر ناقل (عنصر غير مشع) للنويدة المشعة.

وهناك تقنية إشعاعية كيميائية مهمة أخرى، تُسمى تحليل حفز النيوترون. وفي هذه الطريقة يعرض جسم لنيوترونات، لتحويل بعض العناصر فيه إلى عناصر مشعة. تقوم هذه العناصر بعد ذلك، بإطلاق إشعاع له طاقات معينة. وأحد استخدامات هذه الطريقة، هو توضيح مدى موثوقية اللوحات الفنية القديمة. فالدهان المستخدم في الأعمال الفنية القديمة، يختلف في تركيبه عن الدهان الذي يستخدم في اللوحات الفنية الحالية، ولهذا فهو يعطي إشعاعات مختلفة.

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الأنماط الرئيسية للاضمحلال

All radioisotopes are unstable isotopes of elements— that undergo nuclear decay and emit some form of radiation. The radiation emitted can be of several types including alpha, beta, gamma radiation, proton, and neutron emission along with neutrino and antiparticle emission decay pathways.

1. α (alpha) radiation—the emission of an alpha particle (which contains 2 protons and 2 neutrons) from an atomic nucleus. When this occurs, the atom's atomic mass will decrease by 4 units and the atomic number will decrease by 2.

2. β (beta) radiation—the transmutation of a neutron into an electron and a proton. After this happens, the electron is emitted from the nucleus into the electron cloud.

3. γ (gamma) radiation—the emission of electromagnetic energy (such as gamma rays) from the nucleus of an atom. This usually occurs during alpha or beta radioactive decay.

These three types of radiation can be distinguished by their difference in penetrating power.

Alpha can be stopped quite easily by a few centimetres of air or a piece of paper and is equivalent to a helium nucleus. Beta can be cut off by an aluminium sheet just a few millimetres thick and are electrons. Gamma is the most penetrating of the three and is a massless chargeless high-energy photon. Gamma radiation requires an appreciable amount of heavy metal radiation shielding (usually lead or barium-based) to reduce its intensity.


تحليل التفعيل

Glovebox

By neutron irradiation of objects, it is possible to induce radioactivity; this activation of stable isotopes to create radioisotopes is the basis of neutron activation analysis. A high-energy most interesting object which has been studied in this way is the hair of Napoleon's head, which has been examined for its arsenic content.[1]

A series of different experimental methods exist, these have been designed to enable the measurement of a range of different elements in different matrices. To reduce the effect of the matrix it is common to use the chemical extraction of the wanted element and/or to allow the radioactivity due to the matrix elements to decay before the measurement of the radioactivity. Since the matrix effect can be corrected by observing the decay spectrum, little or no sample preparation is required for some samples, making neutron activation analysis less susceptible to contamination.

The effects of a series of different cooling times can be seen if a hypothetical sample that contains sodium, uranium, and cobalt in a 100:10:1 ratio was subjected to a very short pulse of thermal neutrons. The initial radioactivity would be dominated by the 24Na activity (half-life 15 h) but with increasing time the 239Np (half-life 2.4 d after formation from parent 239U with half-life 24 min) and finally the 60Co activity (5.3 yr) would predominate.

تطبقات علم الأحياء

One biological application is the study of DNA using radioactive phosphorus-32. In these experiments, stable phosphorus is replaced by the chemically identical radioactive P-32, and the resulting radioactivity is used in the analysis of the molecules and their behaviour.

Another example is the work that was done on the methylation of elements such as sulfur, selenium, tellurium, and polonium by living organisms. It has been shown that bacteria can convert these elements into volatile compounds,[2] it is thought that methylcobalamin (vitamin B12) alkylates these elements to create the dimethyls. It has been shown that a combination of Cobaloxime and inorganic polonium in sterile water forms a volatile polonium compound, while a control experiment that did not contain the cobalt compound did not form the volatile polonium compound.[3] For the sulfur work, the isotope 35S was used, while for polonium 207Po was used. In some related work by the addition of 57Co to the bacterial culture, followed by isolation of the cobalamin from the bacteria (and the measurement of the radioactivity of the isolated cobalamin) it was shown that the bacteria convert available cobalt into methylcobalamin.

In medicine PET (Positron Emission Tomography) scans are commonly used for diagnostic purposes in. A radiative tracer is injected intravenously into the patient and then taken to the PET machine. The radioactive tracer releases radiation outward from the patient and the cameras in the machine interpret the radiation rays from the tracer. PET scan machines use solid state scintillation detection because of their high detection efficiency, NaI(Tl) crystals absorb the tracer's radiation and produce photons that get converted into an electrical signal for the machine to analyze.[4]

البيئية

Radiochemistry also includes the study of the behaviour of radioisotopes in the environment; for instance, a forest or grass fire can make radioisotopes mobile again.[5] In these experiments, fires were started in the exclusion zone around Chernobyl and the radioactivity in the air downwind was measured.

It is important to note that a vast number of processes can release radioactivity into the environment, for example, the action of cosmic rays on the air is responsible for the formation of radioisotopes (such as 14C and 32P), the decay of 226Ra forms 222Rn which is a gas which can diffuse through rocks before entering buildings[6][7][8] and dissolve in water and thus enter drinking water[9] In addition, human activities such as bomb tests, accidents,[10] and normal releases from industry have resulted in the release of radioactivity.

الصيغة الكيميائية للأكتينيدات

The environmental chemistry of some radioactive elements such as plutonium is complicated by the fact that solutions of this element can undergo disproportionation[11] and as a result, many different oxidation states can coexist at once. Some work has been done on the identification of the oxidation state and coordination number of plutonium and the other actinides under different conditions.[2] This includes work on both solutions of relatively simple complexes[12][13] and work on colloids[14] Two of the key matrixes are soil/rocks and concrete, in these systems the chemical properties of plutonium have been studied using methods such as EXAFS and XANES.[15][3][4]

حركة الغروانيات

While binding of a metal to the surfaces of the soil particles can prevent its movement through a layer of soil, it is possible for the particles of soil that bear the radioactive metal can migrate as colloidal particles through the soil. This has been shown to occur using soil particles labeled with 134Cs, these are able to move through cracks in the soil.[16]

الخلفية المعتادة

Radioactivity is present everywhere on Earth since its formation. According to the International Atomic Energy Agency, one kilogram of soil typically contains the following amounts of the following three natural radioisotopes 370 Bq 40K (typical range 100–700 Bq), 25 Bq 226Ra (typical range 10–50 Bq), 25 Bq 238U (typical range 10–50 Bq) and 25 Bq 232Th (typical range 7–50 Bq).[17]

عمل الكائنات الدقيقة

The action of micro-organisms can fix uranium; Thermoanaerobacter can use chromium(VI), iron(III), cobalt(III), manganese(IV), and uranium(VI) as electron acceptors while acetate, glucose, hydrogen, lactate, pyruvate, succinate, and xylose can act as electron donors for the metabolism of the bacteria. In this way, the metals can be reduced to form magnetite (Fe3O4), siderite (FeCO3), rhodochrosite (MnCO3), and uraninite (UO2).[18] Other researchers have also worked on the fixing of uranium using bacteria [5][6][7], Francis R. Livens et al. (Working at Manchester) have suggested that the reason why Geobacter sulfurreducens can reduce UO2+2 cations to uranium dioxide is that the bacteria reduce the uranyl cations to UO+2 which then undergoes disproportionation to form UO2+2 and UO2. This reasoning was based (at least in part) on the observation that NpO+2 is not converted to an insoluble neptunium oxide by the bacteria.[19]

التعليم

Despite the growing use of nuclear medicine, the potential expansion of nuclear power plants, and worries about protection against nuclear threats and the management of the nuclear waste generated in past decades, the number of students opting to specialize in nuclear and radiochemistry has decreased significantly over the past few decades. Now, with many experts in these fields approaching retirement age, action is needed to avoid a workforce gap in these critical fields, for example by building student interest in these careers, expanding the educational capacity of universities and colleges, and providing more specific on-the-job training.[20]

Nuclear and Radiochemistry (NRC) is mostly being taught at the university level, usually first at the Master- and PhD-degree level. In Europe, substantial effort is being done to harmonize and prepare the NRC education for the industry's and society's future needs. This effort is being coordinated in projects funded by the Coordinated Action supported by the European Atomic Energy Community's 7th Framework Program: The CINCH-II project - Cooperation in education and training In Nuclear Chemistry.


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المراجع

  1. ^ H. Smith, S. Forshufvud and A. Wassén, Nature, 1962, 194(26 May), 725–726
  2. ^ N. Momoshima, Li-X. Song, S. Osaki and Y. Maeda, "Biologically induced Po emission from fresh water", Journal of Environmental Radioactivity, 2002, 63, 187–197
  3. ^ N. Momoshima, Li-X. Song, S. Osaki and Y. Maeda, "Formation and emission of volatile polonium compound by microbial activity and polonium methylation with methylcobalamin", Environmental Science and Technology, 2001, 35, 2956–2960
  4. ^ Saha, Gopal B. (2010). "PET Scanning Systems". Basics of PET Imaging (in الإنجليزية). Springer, New York, NY. pp. 19–39. doi:10.1007/978-1-4419-0805-6_2. ISBN 9781441908049.
  5. ^ Yoschenko VI et al. (2006) Resuspension and redistribution of radionuclides during grassland and forest fires in the Chernobyl exclusion zone: part I. Fire experiments J Envir Radioact 86:143–63 PubMed
  6. ^ Janja Vaupotič and Ivan Kobal, "Effective doses in schools based on nanosize radon progeny aerosols", Atmospheric Environment, 2006, 40, 7494–7507
  7. ^ Michael Durand, Building and Environment, "Indoor air pollution caused by geothermal gases", 2006, 41, 1607–1610
  8. ^ Paolo Boffetta, "Human cancer from environmental pollutants: The epidemiological evidence", Mutation Research/Genetic Toxicology and Environmental Mutagenesis, 2006, 608, 157–162
  9. ^ M. Forte, R. Rusconi, M. T. Cazzaniga and G. Sgorbati, "The measurement of radioactivity in Italian drinking waters". Microchemical Journal, 2007, 85, 98–102
  10. ^ R. Pöllänen, M. E. Ketterer, S. Lehto, M. Hokkanen, T. K. Ikäheimonen, T. Siiskonen, M. Moring, M. P. Rubio Montero and A. Martín Sánchez, "Multi-technique characterization of a nuclearbomb particle from the Palomares accident", Journal of Environmental Radioactivity, 2006, 90, 15–28
  11. ^ Rabideau, S.W., Journal of the American Chemical Society, 1957, 79, 6350–6353
  12. ^ P. G. Allen, J. J. Bucher, D. K. Shuh, N. M. Edelstein, and T. Reich, "Investigation of Aquo and Chloro Complexes of UO22+, NpO2+, Np4+, and Pu3+ by X-ray Absorption Fine Structure Spectroscopy", Inorganic Chemistry, 1997, 36, 4676–4683
  13. ^ David L. Clark, Steven D. Conradson, D. Webster Keogh Phillip D. Palmer Brian L. Scott and C. Drew Tait, "Identification of the Limiting Species in the Plutonium(IV) Carbonate System. Solid State and Solution Molecular Structure of the [Pu(CO3)5]6− Ion", Inorganic Chemistry, 1998, 37, 2893–2899
  14. ^ Jörg Rothe, Clemens Walther, Melissa A. Denecke, and Th. Fanghänel, "XAFS and LIBD Investigation of the Formation and Structure of Colloidal Pu(IV) Hydrolysis Products ", Inorganic Chemistry, 2004, 43, 4708–4718
  15. ^ M. C. Duff, D. B. Hunter, I. R. Triay, P. M. Bertsch, D. T. Reed, S. R. Sutton, G. Shea-McCarthy, J. Kitten, P. Eng, S. J. Chipera, and D. T. Vaniman, "Mineral Associations and Average Oxidation States of Sorbed Pu on Tuff", Environ. Sci. Technol., 1999, 33, 2163–2169
  16. ^ R. D. Whicker and S. A. Ibrahim, "Vertical migration of 134Cs bearing soil particles in arid soils: implications for plutonium redistribution", Journal of Environmental Radioactivity, 2006, 88, 171–188.
  17. ^ "Generic Procedures for Assessment and Response during a Radiological Emergency", International Atomic Energy Agency TECDOC Series number 1162, published in 2000 [1]
  18. ^ Yul Roh, Shi V. Liu, Guangshan Li, Heshu Huang, Tommy J. Phelps, and Jizhong Zhou, "Isolation and Characterization of Metal-Reducing Thermoanaerobacter Strains from Deep Subsurface Environments of the Piceance Basin, Colorado", Applied and Environmental Microbiology, 2002, 68, 6013–6020.
  19. ^ Joanna C. Renshaw, Laura J. C. Butchins, Francis R. Livens, Iain May, John M. Charnock, and Jonathan R. Lloyd, Environ. Sci. Technol., 2005, 39(15), 5657–5660.
  20. ^ Assuring a Future U.S.-Based Nuclear and Radiochemistry Expertise. Board on Chemical Sciences and Technology. 2012. ISBN 978-0-309-22534-2.

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